14

rotaty joint

In our implementation, joint angles are driven toward their desired values by joint actuator torques

generated according to the following proportional-derivative (PD) control law:

t=k_p[!](q_desired-q)-k_d[!]q ^·

where tis the control torque applied at the joint, qis the current joint angle, q_desiredis the desired

joint angle,q
· _{is the relative angular velocity of the rigid links connected by the joint and kpand}

k_dare proportional and derivative control constants.

Figure 2.4- Rotational PD controller for pose control

k_pand k_dspecify the strength of a rotational spring and damper pair which acts as the "muscle"

controlling the joint, as indicated in Figure 2.4.

While this is a simple model, it is sufficient for

our purposes in that it ensures that only internal control forces are used to generate the creature's

motion. The actuator PD gain constants are held fixed for each joint and are considered to be part

of the model specification.

The low-level mechanism for

driving

individual

DOFs

to

their

desired

angles

incorporates

feedback, and is thus an example of closed loop control. The basic pose control mechanism does